如何让我的Python程序在输入无效时退出并显示错误信息？

Question

这是我目前写的代码..

#solving a quadratic equation
#finding conjugate pair of imaginary roots if no real roots

import math
import sys

a = float(input('Enter a: '))
b = float(input('Enter b: '))
c = float(input('Enter c: '))

try:
    # find the discriminant
    d = (b**2)-(4*a*c) 

    if d < 0:
        import cmath #importing complex math to find imaginay roots
        x1 = (-b-cmath.sqrt(d))/(2*a)
        x2 = (-b+cmath.sqrt(d))/(2*a)

        print"This equation gives a conjugate pair of imaginary roots: ", x1, " and", x2

    elif d == 0:
        x = (-b+math.sqrt(d))/(2*a)
        print "This equation has one solutions: ", x

    else:
        x1 = (-b+math.sqrt(d))/(2*a)
        x2 = (-b-math.sqrt(d))/(2*a)
        print "This equation has two solutions: ", x1, " and", x2

except:
    SystemExit

我需要让我的程序能够处理用户输入的非数字数据。之前我尝试过用条件语句（if..elif..else）和异常处理。

我希望程序能显示一个错误信息，并提示用户再试一次。

求助

Answer 1

更新

你可能想把

a = float(input('Enter a: '))
b = float(input('Enter b: '))
c = float(input('Enter c: '))

变成

def get_input(s):
    while True:
        try:
            return float(input('Enter %s: ' % s))
        except ValueError:
            print 'Error: Invalid Input.'


a = get_input('a')
b = get_input('b')
c = get_input('c')